Generalizing the evaluation of medical image processing tools by use of Gabor patterns.

OBJECTIVE: Tools for medical image processing are usually evaluated by observers with radiological experience and with complex tasks. For easing evaluation of filtering and enhancement tools, the observer's task can be generalized. METHODS: By describing aspects of the MCS method (Mammographic Contrast Sensitivity) we illustrate issues of selecting a metric for assessing visual performance, the observer's task and the image material to be used, aiming at a generalization of the design of studies for the evaluation of medical image processing tools. Concerning the metric, we distinguish acuity from contrast sensitivity. With respect to the observer's task, we distinguish tasks of discrimination from those at a higher level of recognition. Finally, we show the advantage of using medical images for evaluating image processing tools by comparing the results for measurements on homogeneous background and mammographic images. RESULTS: The perceptual level of the observer's task and the complexity of the used image material influences the outcome of observer studies, particularly also from crowding effects. The design of a study should minimize the impact of the observer's experience on the outcome. This can be achieved by using non-anatomical, standardized perceptual targets like Gabor patterns, used in the context of medical images. CONCLUSIONS: Understanding the concepts of perception helps designing observer studies that are as complex as required, but at the same time as simple and general as possible. Performing an observer study may be simplified by a study design which does not require radiological experience of the observers, if the study aims at the evaluation of tools that shall support basic perception tasks, such as e.g. contrast enhancement.

[Sighted and blind in one person: a case report and conclusions on the psychoneurobiology of vision]. / Blind und sehend in einer person: schlussfolgerungen zur psychoneurobiologie des sehens.

We present a patient with dissociative identity disorder (DID) who after 15 years of diagnosed cortical blindness gradually regained sight during psychotherapeutic treatment. At first only a few personality states regained vision, whereas others remained blind. This was confirmed by electrophysiological measurement, in which visual evoked potentials (VEP) were absent in the blind personality states but normal and stable in the seeing states. The switch between these states could happen momentarily. As a neural basis of such psychogenic blindness, we assume a top-down modulation of activity in the primary visual pathway, possibly at the level of the thalamus or the primary visual cortex. Therefore VEPs do not allow distinction of psychogenic blindness from organic disruption of the visual pathway. In summary, psychogenic blindness seems to suppress visual information at an early neural stage.

[Methodological principles for optimising functional MR experiments]. / Methodische Grundlagen der Optimierung funktioneller MR-Experimente.

Functional magnetic resonance imaging (fMRI) is one of the most common methods for localising neuronal activity in the brain. Even though the sensitivity of fMRI is comparatively low, the optimisation of certain experimental parameters allows obtaining reliable results. In this article, approaches for optimising the experimental design, imaging parameters and analytic strategies will be discussed. Clinical neuroscientists and interested physicians will receive practical rules of thumb for improving the efficiency of brain imaging experiments.

Calibrated LCD/TFT stimulus presentation for visual psychophysics in fMRI.

Standard projection techniques using liquid crystal (LCD) or thin-film transistor (TFT) technology show drastic distortions in luminance and contrast characteristics across the screen and across grey levels. Common luminance measurement and calibration techniques are not applicable in the vicinity of MRI scanners. With the aid of a fibre optic, we measured screen luminances for the full space of screen position and image grey values and on that basis developed a compensation technique that involves both luminance homogenisation and position-dependent gamma correction. By the technique described, images displayed to a subject in functional MRI can be specified with high precision by a matrix of desired luminance values rather than by local grey value.

Deficits of respiratory-cardiac coupling in heavy drinkers.

1. Physiological evidence of chronic alcohol abuse prior to the onset of clinical signs of alcohol dependence is difficult to obtain The purpose of this study was to search for possible non-invasive indicators for chronic alcohol consumption yielding information in addition to conventional biological markers. 2. The authors investigated the relationship between respiratory-cardiac coupling and blood alcohol concentration (BAC) in male subjects who lost their driver's license from drunk driving. 3. We found that subjects who had a high BAC level (0.16-0.31% at the time of offense) show altered respiratory sinus arrhythmia (RSA) and, in particular, an altered heart-rate response to auditory stimulation and compared them to a control group of social drinkers. Normal subjects showed a pronounced acoustic heart-rate response, i.e., particularly during expiration there was a difference between the interbeat-interval (IBI) traces with and without auditory stimulation. Subjects who had lost their driver's license from drunk driving had an overall severely reduced heart-rate response, that was even absent particularly in the subgroup having high BAC values (0.21-0.31%). The authors also found some evidence that in the latter subgroup IBI, RSA, and acoustic heart-rate responses partially recover after a six-month period of abstinence. 4. Specific parameters of the acoustic heart-rate response are changed in our group of alcohol abusers presumably, due to impairment of vagal function. These parameters may therefore be useful to serve as a non-invasive measure of alcohol abuse.

An explicit no response instead of time-out in automated visual-field testing.

BACKGROUND: To evaluate the effect of response-acquisition technique on psychometric performance in visual-field testing, the conventional one-button yes/time-out method was compared with a two-button yes/no method for responding whether or not the stimulus was detected. There are a number of situations in which the single-button technique leads to ambiguous results. In this study, we thus expected the yes/no method to reduce tendencies towards habituation and automatic responding. Our hypothesis was that the two-button technique could reduce the rate of erroneous responses. METHODS: Luminance-difference sensitivity for bright stimuli (32') on a photopic background was evaluated at 26 locations within the central visual field (30 degrees) using a specially equalised video display unit and a modified 4/2-dB staircase strategy (six reversals, maximum-likelihood threshold estimation). Sixty-one ophthalmologically normal subjects (aged 20-30 years) were examined twice with each method. RESULTS: Mean sensitivities with the two-button yes/no method were found to be, on average, 0.13 dB above those measured with the one-button yes/time-out technique--a difference without clinical relevance. Within-subject variability did not differ between the two methods. However, the less intuitive two-button yes/no method had a slightly higher number of false responses in catch trials. CONCLUSION: Compared to the conventional one-button yes/time-out method, the two-button yes/no method in normal young subjects thus showed little difference in mean sensitivities and equivalent within-subject variabilities. Concerning our initial hypothesis, the yes/no method is of somewhat higher complexity and is not able to reduce the rate of erroneous responses. The one-button yes/time-out method fared a little better in error rate. In summary, the yes/no method is an alternative and additional possibility of response acquisition in visual-field testing, which is worthy of being tested in a clinical study with elderly subjects.

Reaction time in automated kinetic perimetry: effects of stimulus luminance, eccentricity, and movement direction.

PURPOSE: To determine the effects of stimulus eccentricity and luminance level on the reaction time (RT) of young normal volunteers during automated kinetic campimetry. METHODS: We used a specially designed video-campimetric device equipped with a continuous infrared (IR) pupillographic fixation control (Tübingen Computer Campimeter) and recorded reaction times upon presenting horizontally moving small circular stimuli (size 26'; constant angular velocity 2 degrees /s) starting at 16 locations within the central 30 degrees -radius of the visual field. Two different levels of stimulus luminance were used (41.6 cd/m(2) and 110 cd/m(2)), while background luminance was 10 cd/m(2). Each stimulus was presented a total of six times in a randomized order. Subjects were 12 healthy young individuals (aged 21-30 years) with normal ophthalmic examinations. An analysis of variance (ANOVA) was performed on the data. RESULTS: RTs showed considerable inter- and intra-individual variation with individual least squares means (LSM, fitted values of a linear model) ranging from 305 to 454 ms, and residual standard deviation (R.S.D.) 66 ms. Reaction times did not differ significantly as a function of stimulus direction (P>0.6). Higher luminance levels produced significantly reduced reaction times for all stimulus locations and directions (mean reduction: 16 ms; P<0.0001). Reaction times increased with increasing eccentricity, in the mean by 1.8 ms per degree of visual angle, from 365+/-4 ms (S.E.M.) foveally, to 407+/-2 ms at 30 degrees eccentricity; (P<0.0001). CONCLUSIONS: Automated kinetic perimetry should be designed to cope with significant, variable interindividual response characteristics. Other stimulus related factors, such as eccentricity or luminance level, have a significant but comparatively small effect on reaction time within the central 30 degrees -radius visual field in healthy young individuals.

Acute effects of alcohol on divided and covert attention in men.

RATIONALE: While several studies identified divided attention to be sensitive to alcohol effects, the impact of alcohol on covert visual attention is still not clear, despite the latter's important role in perception. OBJECTIVES: The study tests the effect of acute moderate doses of alcohol on divided and covert attention in right-handed, male volunteers. METHODS: The design of the study involved a double-blind trial with an alcohol and a placebo condition; measurements were taken before and after an oral dose of 0.6 g/kg alcohol versus placebo. In the divided-attention task, simultaneous visuo-spatial and auditory stimulation was applied. In a test of covert attention, subjects had to shift their attentional focus according to a central cue, from one location in the visual field to another. RESULTS: Under the divided-attention condition, reaction times were significantly prolonged after alcohol ingestion compared to placebo. Covert attention pre-post change was also significantly different between the alcohol and placebo groups. There is a reduction of false-cueing disturbance for left-appearing stimuli under moderate alcohol but an increase of disturbance for rightward stimuli, i.e. we found a lateralised pattern of reaction for spatial orienting. In the placebo group, no significant differences in right-left performance were obtained. CONCLUSION: The results suggest that sensory-attentional mechanisms play a key role in altered visual perceptual performance after alcohol ingestion. Furthermore, differences between the right and left visual field in the cued target-detection task indicate that alcohol exerts an influence on right-hemispheric attentional priming.

Converting between measures of slope of the psychometric function.

The psychometric function's slope provides information about the reliability of psychophysical threshold estimates. Furthermore, knowing the slope allows one to compare, across studies, thresholds that were obtained at different performance criterion levels. Unfortunately, the empirical validation of psychometric function slope estimates is hindered by the bewildering variety of slope measures that are in use. The present article provides conversion formulas for the most popular cases, including the logistic, Weibull, Quick, cumulative normal, and hyperbolic tangent functions as analytic representations, in both linear and log coordinates and to different log bases, the practical decilog unit, the empirically based interquartile range measure of slope, and slope in a d' representation of performance.

Invariance of the psychometric function for character recognition across the visual field.

The psychometric function for recognition of singly presented digits as a function of digit contrast was measured at 2 degrees steps across the horizontal meridian of the visual field, under monocular and binocular viewing conditions. A maximum-likelihood staircase procedure was used in a 10-alternative forced-choice recognition paradigm to gather the data Both the Weibull and the logistic psychometric functions provide excellent fits to the observed data. The slopes of these functions at their point of inflection ranged from 4.0 to 5.0 proportion-correct/log10-unit contrast, for both monocular and binocular viewing and for all loci in the visual field. These slope values correspond to short-term measurements (around 30 trials, or 1 min) and do not include performance variations of longer duration; the latter are estimated to increase slope by a factor of about 1.5. A single psychometric function shape, centered around a threshold value, therefore describes recognition performance at all retinal loci and binocularity. An empirical comparison of slope results across the literature shows that the function's slope is about twice that reported for a number of detection tasks. The comparison of recognition contrast thresholds, percentage correct values, and other performance measures across studies requires the knowledge of the psychometric function's slope, and our results thus provide a firm basis for the study of low-contrast character recognition.

Assessing spatial vision - automated measurement of the contrast-sensitivity function in the hooded rat.

The contrast-sensitivity function (CSF) provides a concise and thorough description of an organism's spatial vision; it is widely used to describe vision in animals and humans, to track developmental changes in vision, and to compare vision among different species. Despite the predominance of rats in neuroscience research, their vision is not thoroughly studied due to the complexity of psychophysical measurement and a generally held notion that rat vision is poor. We therefore designed an economical and rapid method to assess the hooded rat's CSF, using a computer monitor to display stimuli and an infrared touch screen to record responses. A six-alternative forced-choice task presented trials in which a sine-wave grating (S+), varying in spatial frequency and contrast, was displayed at different locations along with five gray stimuli (S-). Nose pokes to the S+ but not the S- produced water reinforcers. Contrasts were tested at each spatial frequency with a simple adaptive procedure until stimulus detection fell below chance. Psychometric functions were obtained by maximum-likelihood fitting of a logistic function to the raw data, obtaining the threshold as the function's point of inflection. As in previous studies with rats, CSFs showed an inverse-U shape with peak sensitivity at 0.12 cyc/deg and acuity just under 1 cyc/deg. The results indicate the present computer-controlled behavioral testing device is a precise and efficient instrument to assess spatial visual function in rats.

Auditory temporal-order judgement is impaired in patients with cortical lesions in posterior regions of the left hemisphere.

Auditory temporal-order judgement was investigated in patients suffering from unilateral focal brain lesions, localized in anterior or posterior regions of the left hemisphere (LH) (resulting in non-fluent or fluent aphasia, respectively), or in predominantly subcortical regions of this hemisphere (without aphasic syndromes) and in anterior or posterior regions of the right hemisphere. The temporal order threshold was measured as the minimum time interval between two clicks presented consecutively and binaurally via headphones (one to each ear) that was necessary for a subject to indicate the temporal order of the two stimuli. Only the patient group with fluent aphasia showed a significantly increased mean temporal-order threshold as compared to the controls. Our results indicate that fine temporal resolution for auditory stimuli is predominantly associated with posterior regions of the LH.

Automobile driving performance of brain-injured patients with visual field defects.

The purpose of this study was to examine whether patients with visual field defects resulting from cerebral injury are handicapped in their driving ability, because visual field loss as assessed in standard perimetry is often the basis for withdrawal of a person's driving license. Driving performance was tested on a driving simulator to obtain standardized results and for safety reasons. The visual field was assessed both with standard automated perimetry and computer-based, high-resolution, qualitative perimetry. We investigated nine patients with purely cerebral field defects (mostly homonymous binocular defects) who had no further neuropsychological or ophthalmological deficits. Their performance (driving speed, reaction time, and driving error rate) was compared with that of a control group of ten subjects. We found no differences in any of the tested parameters between the visually impaired subjects and the normal participants. This suggests that individuals with visual field defects, including those who suffer from homonymous hemianopia, may perform as adequately as normal individuals in realistic driving scenarios. The perimetrically assessed visual field may, thus, be of limited value for the prediction of driving safety, and we conclude that patients who have field defects should not summarily be denied a driving license.

Fitting the psychometric function.

A constrained generalized maximum likelihood routine for fitting psychometric functions is proposed, which determines optimum values for the complete parameter set--that is, threshold and slope--as well as for guessing and lapsing probability. The constraints are realized by Bayesian prior distributions for each of these parameters. The fit itself results from maximizing the posterior distribution of the parameter values by a multidimensional simplex method. We present results from extensive Monte Carlo simulations by which we can approximate bias and variability of the estimated parameters of simulated psychometric functions. Furthermore, we have tested the routine with data gathered in real sessions of psychophysical experimenting.

Raster-scan cathode-ray tubes for vision research--limits of resolution in space, time and intensity, and some solutions.

Raster-based cathode-ray tubes (CRTs) are increasingly used for stimulus presentation. While very flexible, their design based on consumer electronics can limit their value in vision research. Here their limitations of resolution in time, space, intensity and wavelength are systematically compiled. Often, ingenious ideas can circumvent such limitations for specific experiments. Some ad-hoc solutions, as well as the more general techniques of dithering and anti-aliasing, are presented.

Contrast-dependent dissociation of visual recognition and detection fields.

Seeing an object 'as something' is different from simply seeing it (see Watanabe, S., 1985, Pattern Recognition: Human and Mechanical, John Wiley). This distinction between recognition and detection often goes unnoticed in physiology and clinical practice, where visual performance is characterized in terms of acuity, visual field and contrast sensitivity. The corresponding functions of stimulus detection are consistent with the neural projection properties from the retina to the striate cortex, i.e. the 'cortical magnification theory'. Yet recognition performance for characters (Strasburger, H. et al., 1994, Eur. J. Neurosci., 6, 1583-1588) and grey-level patterns (Jüttner, M. and Reutschler, I., 1996, Vision Res., 36, 1007-1022) does not fit into this scheme. Here we show that this discrepancy results in the dissociation of visual recognition and detection fields, which is dramatic at low pattern contrast. Form proper can be appreciated exclusively within the much narrower field of recognition, the window of visual intelligence. Its function is, at low contrast, probably mediated by the magnocellular pathway and at all contrasts is determined by the processing characteristics of higher stages of the ventral visual pathway.

Objective measurement of contrast sensitivity and visual acuity with the steady-state visual evoked potential.

Since the appearance of Campbell and Maffei's and Harter and White's reports it has been well established that the visual evoked potential (VEP) can be used to predict psychophysical contrast sensitivity and visual acuity and is thus suited as an objective technique to assess these fundamental aspects of vision. Nevertheless, the technique has not become a standard diagnostic tool, being too time-consuming to apply and suffering from variable reliability under pathological visual conditions. In addition, there are problems of reliability in normal subjects. By using an unconventional stimulus--temporally sinusoidal 16-Hz on-off modulation of sinewave gratings--we demonstrated that these problems can be alleviated in normal subjects. This stimulus avoids the low signals in the visible range that frequently occur with conventional pattern-reversal stimuli, it leads to high correspondence between normal observers, and it is much faster to apply than are transient VEPs. Initial applications of this stimulus to amblyopes yielded promising results. The steady-state VEP could consequently turn into a viable diagnostic procedure in disturbances of visual contrast perception.